Cell separation techniques have important potential application in cancer therapies, autoimmune disease therapies, and improved diagnostics. For example, cell affinity devices can be used in extracorporeal therapies that may involve the selective isolation, augmentation, and reintroduction to the host of a specific subset population of cells.
Affinity separation of cells refers to process techniques where a particular subset of a population of cells are bound to support surfaces by means of ligands with specific affinity to molecules or structures on the cell membrane. Cells which lack the membrane molecules or structures are not bound to the support surface and can be removed from the population to effect a separation. Cell affinity techniques have been used widely since Wigzell's description of such a process in 1969 [Wigzell and Anderson (1969) J. Exp. Med. 129:23].
Affinity separation processes are commonly used either to deplete cell subpopulations from a mixture or to positively select a specific population from a mixture. The depletion process is much simpler because the bound cells are simply discarded leaving the desired cells behind. Positive selection is much more difficult both because the desired cells are bound to the support and must be removed without damaging them and because a certain proportion of the undesirable cells bind nonspecifically to the affinity surface and contaminate the desired collected cells.
Affinity cell depletion techniques have found some important applications. Researchers prepare specific cell subpopulations for study by systematically depleting a mixture of various subpopulations of cells. For example, Treleavan et al. (Treleavan et al., 1984, Lancet 1:70-73) have demonstrated that the concentration of neuroblastoma cells in a bone marrow preparation can be reduced by a factor of about 10.sup.6 using multiple depletions with antibody-coated magnetic beads.
Two examples of positive selection techniques are those described by Berenson et al. (J. Immunol. Methods, 1986, 91:179-187) and Gaudernack et al., J. Immunol. Methods, 1986, 91:179-187. Berenson et al. bind biotinylated antibodies to target cells and pass them through a column packed with avidin-coated beads, thereby recovering 64% of a population of human bone marrow cells at a final concentration of 73% when the initial concentration was 7%. Gaudernack et al. use antibody-coated magnetic beads to collect a certain subset of T cells. The initial concentration was 30%, and the positively selected population was 96%; the yield is not mentioned. These purities are not adequate for a large number of attractive applications, such as stem cell transplants, or the preparation of subpopulations for cell biology or immunology studies.
Repeated contacting of cells has been shown to be effective in the depletion of cells from mixtures, but repeated contacting of cells for positive selection of subpopulations has not been reported. This is not surprising since theories of the mechanism of affinity cell binding predict no advantage with multiple stages (Hertz et al., 1985, Biotech. and Bioeng., 27:603-612; Bell et al., 1984, Biophys. J. 45; Grinnell, 1978, Intl. Rev. Cytology, 53:65-144). There remains a need to be able to recover cells with higher yields and higher purities.